1. Field of the Invention
The present invention relates to global location satellite systems and in particular to automatic frequency control for GLONASS or GPS.
2. Related Art
GPS (global positioning system) and GLONASS (global navigation satellite system) are radio-based satellite systems in operation today. To provide global coverage, GPS uses between 24-32 satellites. Assuming the minimum number of 24 satellites, 4 satellites are deployed in each of six orbits. The six orbital planes' ascending nodes are separated by 60 degrees. In this configuration, a minimum of six satellites should be in view from any given point at any time. To provide global coverage, GLONASS includes 24 satellites, wherein 21 satellites can be used for transmitting signals and 3 satellites can be used as spares. The 24 satellites are deployed in three orbits, each orbit having 8 satellites. The three orbital planes' ascending nodes are separated by 120 degrees. In this configuration, a minimum of five satellites should be in view from any given point at any time.
Both GPS and GLONASS broadcast two signals: a coarse acquisition (C/A code) signal and a precision (P code) signal. In general, global position devices, called receivers herein, lock onto the C/A transmission and not the P transmission. The P transmission is much longer than the C/A transmission and therefore is impractical to lock onto, e.g. by using synchronization. Once a lock is established via C/A transmission, the C/A transmission itself can provide a quick P lock.
The C/A codes for GPS and GLONASS, which can be generated as a modulo-2 sum of two maximum length shift register sequences, are selected for good cross-correlation properties. Each GPS satellite transmits its own unique C/A code, which has an identifiable pseudo-random noise code number (PRN#). In contrast, each GLONASS satellite transmits the same C/A code, and is identified by its channel number (CHN#).
The C/A code includes navigation data, which provides information about the exact location of the satellite, the offset and drift of its on-board atomic clock, and information about other satellites in the system. In GPS, the C/A format for the navigation data includes words, frames, and subframes. The words are 30 bits long; ten words form one subframe; and five subframes form one frame. In GPS, the C/A code is 1023 bits long, is transmitted at 1.023 Mbps, and therefore has a repetition period of 1 ms. In GLONASS, the C/A format is strings, wherein each string includes 1.7 sec of navigation data and 0.3 sec of a time mark sequence. Notably, the C/A code in GLONASS is 511 bits long, is transmitted at 511 kbps, and therefore has the same code repetition period (i.e. 1 ms) as GPS.
FIG. 1A illustrates a GLONASS string 100 including 85 data bits in bi-binary code and 30 bits of time mark. FIG. 1B shows the 85 data bits in both relative code (101) (having a 20 ms period) before encoding and in bi-binary code (102) after encoding. Note that the encoding is a modulation of the relative code by a meander sequence (103), which changes polarity every 10 ms, as shown by clock pulses 104). Therefore, bi-binary code 102 has an effective data bit duration of 10 ms.
FIG. 1B shows the 30 bits of the time mark (105) aligned with the 85 data bits for comparison. As shown, each of the 30 time mark bits is 10 ms long. The 30 bits of the time mark pattern are [111110001101110101000010010110]. The time mark is provided to facilitate time synchronization of the satellite's atomic clock to the receiver's local clock.
With the advent of GLONASS satellites now being available to provide position information, it is desirable to have a system that includes the capability of using GPS and/or GLONASS signals for position determination. Therefore, a need arises for a method for receiving both GPS and GLONASS signals.